Coaxial multiple reflection time-of-flight mass spectrometer

ABSTRACT

The present invention relates to an apparatus and method for analyzing ions including an ion accelerator and one or more reflectrons positioned with respect to one another such that ions can be reflected back and forth between therebetween. The ion accelerator acts both to provide the initial acceleration of ions received from an ion source and to reflect these ions in the subsequent mass analysis. The reflectrons act only reflect ions in such a manner that all ions of a given mass-to-charge ratio have substantially the same flight time through the analyzer. During ion analysis, ions are reflected back and forth between the accelerator and the reflectrons multiple times, until, at the conclusion of the ion analysis, the accelerator is rapidly deenergized so as to allow the ions to pass through the accelerator and into a detector. Alternatively, the ions may be deflected into a detector using electrostatic deflection plates or may pass through one of the reflectrons into a detector by deenergizing one of the reflectrons. By reflecting the analyte ions back and forth between the accelerator and the reflectron many times, a much longer flight path can be achieved than previously used according to the prior art Consequently, the mass resolving power of the spectrometer disclosed can be substantially greater than could otherwise be achieved.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is a continuation-in-part of application Ser. No08/866,134, filed May 30, 1997.

TECHNICAL FIELD OF THE INVENTION

[0002] The present invention relates generally to the mass spectroscopicanalysis of chemical samples and more particularly to time-of-flightmass spectrometry. A means and method are described for the analysis ofionized species in a spectrometer comprising two or more reflectingdevices such that ions can be reflected back and forth a plurality oftimes therebetween.

BACKGROUND OF THE PRESENT INVENTION

[0003] The present invention relates in general to ion beam handling inmass spectrometers and more particularly to a means of focusing ions intime-of-flight mass spectrometers (TOFMS). The apparatus and method ofmass analysis described herein is an enhancement of the techniques thatare referred to in the literature relating to mass spectrometry.

[0004] The analysis of ions by mass spectrometers is important, as massspectrometers are instruments that are used to determine the chemicalstructures of molecules. In these instruments, molecules becomepositively or negatively charged in an ionization source and the massesof the resultant ions are determined in vacuum by a mass analyzer thatmeasures their mass/charge (m/q) ratio. Mass analyzers come in a varietyof types, including magnetic field (B), combined (double-focusing)electrical (E) and magnetic field (B), quadrupole (Q), ion cyclotronresonance (ICR), quadrupole ion storage trap, and time-of-flight (TOF)mass analyzers, which are of particular importance with respect to theinvention disclosed herein. Each mass spectrometric method has a uniqueset of attributes. Thus, TOFMS is one mass spectrometric method thatarose out of the evolution of the larger field of mass spectrometry.

[0005] The analysis of ions by TOFMS is, as the name suggests, based onthe measurement of the flight times of ions from an initial position toa final position. Ions which have the same initial kinetic energy butdifferent masses will separate when allowed to drift through a fieldfree region.

[0006] Ions are conventionally extracted from an ion source in smallpackets. The ions acquire different velocities according to themass-to-charge ratio of the ions. Lighter ions will arrive at a detectorprior to high mass ions. Determining the time-of-flight of the ionsacross a propagation path permits the determination of the masses ofdifferent ions. The propagation path may be circular or helical, as incyclotron resonance spectrometry, but typically linear propagation pathsare used for TOFMS applications.

[0007] TOFMS is used to form a mass spectrum for ions contained in asample of interest. Conventionally, the sample is divided into packetsof ions that are launched along the propagation path using apulse-and-wait approach. In releasing packets, one concern is that thelighter and faster ions of a trailing packet will pass the heavier andslower ions of a preceding packet. Using the traditional pulse-and-waitapproach, the release of an ion packet as timed to ensure that the ionsof a preceding packet reach the detector before any overlap can occur.Thus, the periods between packets is relatively long. If ions are beinggenerated continuously, only a small percentage of the ions undergodetection. A significant amount of sample material is thereby wasted.The loss in efficiency and sensitivity can be reduced by storing ionsthat are generated between the launching of individual packets, but thestorage approach carries some disadvantages.

[0008] Resolution is an important consideration in the design andoperation of a mass spectrometer for ion analysis. The traditionalpulse-and-wait approach in releasing packets of ions enables resolutionof ions of different masses by separating the ions into discerniblegroups. However, other factors are also involved in determining theresolution of a mass spectrometry system. “Space resolution” is theability of the system to resolve ions of different masses despite aninitial spatial position distribution within an ion source from whichthe packets are extracted. Differences in starting position will affectthe time required for traversing a propagation path. “Energy resolution”is the ability of the system to resolve ions of different mass despitean initial velocity distribution. Different starting velocities willaffect the time required for traversing the propagation path.

[0009] In addition, two or more mass analyzers may be combined in asingle instrument to form a tandem mass spectrometer (MS/MS, MS/MS/MS,etc.). The most common MS/MS instruments are four sector instruments(EBEB or BEEB), triple quadrupoles (QQQ), and hybrid instruments (EBQQor BEQQ). The mass/charge ratio measured for a molecular ion is used todetermine the molecular weight of a compound. In addition, molecularions may dissociate at specific chemical bonds to form fragment ions.Mass/charge ratios of these fragment ions are used to elucidate thechemical structure of the molecule. Tandem mass spectrometers have aparticular advantage for structural analysis in that the first massanalyzer (MS1) can be used to measure and select molecular ion from amixture of molecules, while the second mass analyzer (MS2) can be usedto record the structural fragments. In tandem instruments, a means isprovided to induce fragmentation in the region between the two massanalyzers. The most common method employs a collision chamber filledwith an inert gas, and is known as collision induced dissociation CID.Such collisions can be carried out at high (5-10 keV) or low (10-100 eV)kinetic energies, or may involve specific chemical (ion-molecule)reactions. Fragmentation may also be induced using laser beams(photodissociation), electron beams (electron induced dissociation), orthrough collisions with surfaces (surface induced dissociation). It ispossible to perform such an analysis using a variety of types of massanalyzers including TOF mass analysis.

[0010] In a TOFMS instrument, molecular and fragment ions formed in thesource are accelerated to a kinetic energy:

qV=½mv²  (1)

[0011] where q is the elemental charge, V is the potential across thesource/accelerating region, m is the ion mass, and v is the ionvelocity. These ions pass through a field-free drift region of length Lwith velocities given by equation 1. The time required for a particularion to traverse the drift region is directly proportional to the squareroot of the mass/charge ratio:

t=L(m/2qV)^(1/2)  (2)

[0012] Conversely, the mass/charge ratios of ions can be determined fromtheir flight times according to the equation:

m/e=αt ²+β  (3)

[0013] where α and β are constants which can be determinedexperimentally from the flight times of two or more ions of knownmass/charge ratios.

[0014] Generally, TOF mass spectrometers have limited mass resolution.This arises because there may be uncertainties in the time that the ionswere formed (time distribution), in their location in the acceleratingfield at the time they were formed (spatial distribution), and in theirinitial-kinetic energy distributions prior to acceleration (energydistribution).

[0015] The first commercially successful TOFMS was based on aninstrument described by Wiley and McLaren in 1955 (Wiley, W. C.;McLaren, I. H., Rev. Sci. Instrumen. 26 1150 (1955)). That instrumentutilized electron impact (EI) ionization (which is limited to volatilesamples) and a method for spatial and energy focusing known as time-lagfocusing. The simplest form of the Wiley-McLaren instrument is depictedin FIG. 1. The instrument consists, in part, of an ion accelerator, adetector, and a field free drift region between the accelerator and thedetector. At the beginning of an analysis, ions are located in theaccelerator near plane P₁—the “object plane”. The ions initially havenear thermal kinetic energies. To begin the analysis, an electricalpotential is applied to the accelerator. The electric field in theaccelerator accelerates ions toward a detector which resides at planeP₂—the “image plane”. For the purpose of the present discussion, it isassumed that ions are accelerated in a single region of the acceleratorand that the electric field strength is uniform throughout this region.

[0016] As was first derived by Wiley and Maclaren, ions of a givenmass-to-charge ratio (m/q) starting at a variety of positions near theobject plane will all arrive at the image plane at substantially thesame time. This effect is referred to by Wiley and Maclaren as “spacefocusing”. Notice that if s is the distance between the object plane,P₁, and the end of the accelerator, then D, the distance between theimage plane, P₂, and the end of the accelerator, will be equal to 2 s.Placing the detector at the image plane will result in the optimal spacefocusing and therefore the highest mass resolution.

[0017] Wiley and Maclaren also described the use of two consecutiveacceleration regions whereby an image plane may be formed farther fromthe accelerator and therefore provide higher mass resolution. It is this“two stage” acceleration instrument that was commercialized. In thecommercialized instrument, molecules are first ionized by a pulsed (1-5microsecond) electron beam. Spatial focusing was accomplished using twostages of ion acceleration. In the first stage, a low voltage (−150 V)drawout pulse is applied to the source region that compensates for ionsformed at different locations, while the second stage completes theacceleration of the ions to their final kinetic energy (−3 keV ). Ashort time-delay (1-7 microseconds) between the ionization and drawoutpulses compensates for different initial kinetic energies of the ions,and is designed to improve mass resolution. Because this method requireda very fast (40 ns) rise time pulse in the source region, it wasconvenient to place the ion source at ground potential, while the driftregion floats at −3 kV. The instrument was commercialized by BendixCorporation as the model NA-2, and later by CVC Products (Rochester,N.Y.) as the model CVC-2000 mass spectrometer The instrument has apractical mass-range of 400 daltons and a mass resolution of {fraction(1/300)}, and is still commercially available.

[0018] There have been a number of variations on this instrument. Muga(TOFTEC, Gainsville) has described a velocity compaction technique forimproving the mass resolution (Muga velocity compaction). Chatfield etal. (Chatfield FT-TOF) described a method for frequency modulation ofgates placed at either end of the flight tube, and Fouriertransformation to the time domain to obtain mass spectra. This methodwas designed to improve the duty cycle.

[0019] Cotter et al. (VanBreeman, R. B.: Snow, M.: Cotter, R. J., Int.J. Mass Spectrom. Ion Phys. 49 (1983) 35.; Tabet, J. C.; Cotter, R. J.,Anal. Chem. 56 (1984) 1662; Olthoff, J. K.; Lys, I.: Demirev, P.:Cotter, R. J., Anal. Instrumen. 16 (1987) 93) modified a CVC 2000time-of-flight mass spectrometer for infrared laser desorption ofinvolatile biomolecules, using a Tachisto (Needham, Mass.) model 215Gpulsed carbon dioxide laser. This group also constructed a pulsed liquidsecondary time-of-flight mass spectrometer (liquid SIMS-TOF) utilizing apulsed (1-5 microsecond) beam of 5 keV cesium ions, a liquid samplematrix, a symmetric push/pull arrangement for pulsed ion extraction(Olthoff, J. K.; Cotter, R. J., Anal. Chem. 59 (1987) 999-1002.;Olthoff, J. K.; Cotter, R. J., Nucl. Instrum. Meth. Phys. Res.B-26(1987) 566-570. In both of these instruments, the time delay rangebetween ion formation and extraction was extended to 5-50 microseconds,and was used to permit metastable fragmentation of large molecules priorto extraction from the source. This in turn reveals more structuralinformation in the mass spectra.

[0020] The plasma desorption technique introduced by Macfarlane andTorgerson in 1974 (Macfarlane, R. D.; Skowronski, R. P.; Torgerson, D.F., Biochem. Biophys. Res Commoun. 60 (1974) 616.) formed ions on aplanar surface placed at a voltage of 20 kV. Since there are no spatialuncertainties, ions are accelerated promptly to their final kineticenergies toward a parallel, grounded extraction grid, and then travelthrough a grounded drift region. High voltages are used, since massresolution is proportional to U_(o)/eV, where the initial kineticenergy, U_(o) is of the order of a few electron volts. Plasma desorptionmass spectrometers have been constructed at Rockefeller (Chait, B. T.;Field, F. H., J. Amer. Chem. Soc. 106 (1984) 193), Orsay (LeBeyec, Y.;Della Negra, S.; Deprun, C.; Vigny, P.; Giont, Y. M., Rev. Phys. Appl.15 (1980) 1631), Paris (Viari, A.; Ballini, J. P.; Vigny, P.; Shire, D.;Dousset, P., Biomed. Environ. Mass Spectrom, 14 (1987) 83), Upsalla(Hakansson, P.; Sundqvist B., Radiat. Eff. 61 (1982) 179) and Darmstadt(Becker, O.; Furstenau, N.; Krueger, F. R.; Weiss, G.; Wein, K., Nucl.Instrum. Methods 139 (1976) 195). A plasma desorption time-of-flightmass spectrometer has been commercialized by BIO-ION Nordic (Upsalla,Sweden). Plasma desorption utilizes primary ion particles with kineticenergies in the MeV range to induce desorption/ionization. A similarinstrument was constructed at Manitobe (Chain, B. T.; Standing, K. G.,Int. J. Mass Spectrum. Ion Phys. 40 (1981) 185) using primary ions inthe keV range, but has not been commercialized.

[0021] Matrix-assisted laser desorption, introduced by Tanaka et al.(Tanaka, K.; Waki, H.; Ido, Y.; Akita, S.; Yoshida, Y.; Yoshica, T.,Rapid Commun. Mass Spectrom. 2 (1988) 151) and by Karas and Hillenkamp(Karas, M.; Hillenkamp, F., Anal. Chem. 60 (1988) 2299) utilizes TOFMSto measure the molecular weights of proteins in excess of 100,000daltons. An instrument constructed at Rockefeller (Beavis, R. C.; Chait,B. T., Rapid Commun. Mass Spectrom. 3 (1989) 233) has beencommercialized by VESTEC (Houston, Tex.), and employs prompt two-stageextraction of ions to an energy of 30 keV.

[0022] Time-of-flight instruments with a constant extraction field havealso been utilized with multi-photon ionization, using short pulselasers.

[0023] The instruments described thus far are linear time-of-flightspectrometers. That is, there is no additional focusing after the ionsare accelerated and allowed to enter the drift region. Two approaches toadditional energy focusing have been utilized: those which pass the ionbeam through an electrostatic energy filter and those which use an ionmirror.

[0024] The reflectron (or ion mirror) was first described by Mamyrin(Mamyrin, B. A.; Karatajev. V. J.; Shmikk, D. V.; Zagulin, V. A., Sov.Phys., JETP 37 (1973) 45). As depicted in FIG. 2, the operation of thereflectron is in effect the same as that of the ion acceleratordiscussed above. Ions are assumed to start at an object or image planelocated at P₂. Ions are assumed to start at plane P₂ having already beenaccelerated to their full kinetic energy and moving toward thereflectron. After having traveled some distance, D₂, in a field freeregion, the ions enter the reflectron. The electrostatic field withinthe energized reflectron slows the ions to a stop at a distance s fromthe entrance of the reflectron. Ions are then re-accelerated towardimage plane P₃ and to their original kinetic energy by the reflectron'selectrostatic field. After exiting the reflectron, the ions travel adistance D₂ to image plane P₃. Within a certain kinetic energy range,all ions of a given m/q, having started at plane P₂ simultaneously, willarrive at image is plane P₃ at substantially the same time. Improvedmass resolution results from the fact that ions with larger kineticenergies must penetrate the reflecting field more deeply before beingturned around. These faster ions then catch up with the slower ions atthe detector and are thus temporally focused.

[0025] For the purpose of the present discussion, it is assumed thations are accelerated in a single region of the reflectron and that theelectric field strength is uniform throughout this region. In such acase, the relationship between D₁, D₂, and s is given by:

D ₁ +D ₂=4s  (4)

[0026] If D₁=D₂=D then as in the discussion of the Wiley-Maclarenaccelerator above, D=2 s.

[0027] As with the Wiley-Maclaren accelerator, the reflectron mightconsist of more than one acceleration “stage”. Such multistagereflectrons have been discussed extensively in the technical literature.See, for example, U. Boesl, R. Weinkauf, and E. W. Schlag, Int. J. MassSpectrom. Ion Process., 112, 121(1992). Multistage reflectrons have theadvantage that they can temporally focus ions of a broader range ofkinetic energies.

[0028] The Wiley-Maclaren accelerator and Mamyrin reflectron may becombined in a single instrument as depicted in FIG. 3. Here, ions startat object plane, P₁, in a single stage accelerator. The ions areaccelerated and space focused to image plane, P₂. As discussed withrespect to FIG. 1, due to space focusing all ions of a given m/q passthrough plane P₂ at substantially the same time. From this point, thedistribution of ion kinetic energies would result in a temporaldefocusing of the ions and a loss in mass resolution. However, imageplane P₂ may be treated as the starting point for ions passing throughthe reflectron. As discussed with respect to FIG. 2, the reflectron canfocus ions from image plane P₂ to image plane P₃. If a detector isplaced at P₃ then all ions of a given m/q will be temporally focused sothat they arrive at the detector at substantially the same time and willthereby provide the optimum mass resolution.

[0029] Reflectrons were used on the laser microprobe instrumentintroduced by Hillenkamp et al. (Hillenkamp, F.; Kaufmann, R.; Nitsche,R.; Unsold, E., Appl. Phys. 8 (1975) 341) and commercialized by LeyboldHereaus as the LAMMA (LAser Microprobe Mass Analyzer). A similarinstrument was also commercialized by Cambridge Instruments as the IA(Laser Ionization Mass Analyzer). Benninghoven (Benninghoven reflectron)has described a SIMS (secondary ion mass spectrometer) instrument thatalso utilizes a reflectron, and is currently being commercialized byLeybold Hereaus. A reflecting SIMS instrument has also been constructedby Standing (Standing, K. G.; Beavis, R.; Bollbach, G.; Ens, W.;Lafortune, F.; Main, D.; Schueler, B.; Tang, X.; Westmore, J. B., Anal.Instrumen. 16 (1987) 173).

[0030] Lebeyec (Della-Negra, S.; Lebeyec, Y., in Ion Formation fromOrganic Solids IFOS III, ed. by A. Benninghoven, pp 42-45,Springer-Verlag, Berlin (1986)) described a coaxial reflectrontime-of-flight mass spectrometer that reflects ions along the same pathin the drift tube as the incoming ions, and records their arrival timeson a channelplate detector with a centered hole that allows passage ofthe initial (unreflected) beam. This geometry was also utilized byTanaka et al. (Tanaka, K.; Waki, H.; Ido, Y.; Akita, S.; Yoshida, T.,Rapid Comun. Mass Spectrom. 2 (1988) 151) for matrix assisted laserdesorption. Schlag et al. (Grotemeyer, J.; Schlag, E. W., Org. MassSpectrom. 22 (1987) 758) have used a reflectron on a two-laserinstrument. The first laser is used to ablate solid samples, while thesecond laser forms ions by multiphoton ionization. This instrument iscurrently available from Bruker. Wollnik et al. (Grix., R.; Kutscher,R.; Li, G.; Gruner, U.; Wollnik, H., Rapid Commun. Mass Spectrom. 2(1988) 83) have described the use of reflectrons in combination withpulsed ion extraction, and achieved mass resolutions as high as 20,000for small ions produced by electron impact ionization.

[0031] A dual-reflectron time-of-flight mass spectrometer has beenpreviously described by Cotter et al.(Cotter, R. J. and Cornish, T. J.;U.S. Pat. No. 5,202,563 and Cornish, T. J., and Cotter, R. J., Time ofFlight Mass Spectrometry, R. J. Cotter ed., American Chemical Society,Washington, D. C., 1994). The instrument described comprises an ionsource wherein ions are generated and then accelerated towards a firstreflectron. An electrostatic field generated by the energized reflectronreflects ions towards a second reflectron. Similarly, the secondreflectron reflects ions toward an ion detector. Cotter et al.demonstrated that in one particular instance the mass resolving power ofthe spectrometer observed when using the instrument as described aboveis about double that observed when using only a single reflectron.Notably, however, the spectrometer described by Cotter et al. is limitedto two reflections as only two reflectrons are used and these arepositioned so that ions follow a Z shaped trajectory through theinstrument. Also, notable is the fact that neither of the reflectronscan be pulsed on or off in a microsecond time frame.

[0032] Additionally, Mamyrin et al. U.S. Pat. No. 4,072,862 discloses atime-of-flight mass spectrometer whose analyzer chamber accommodates apulsed ion source, an ion detector and an ion reflecting system, alldisposed on one and the same ion optical axis. Mamyrin's prior artspectrometer is depicted in FIG. 4. Parts of the spectrometer accordingto the present invention resemble this arrangement superficially,however, as will be seen below, the present invention has somesignificant differences with regard to both means and method. Notice inthe case of Mamyrin that ions are generated in a source which isintegrated into the mass analyzer. The ion detector and the ionreflecting system described in Mamyrin et al. are disposed on oppositesides of the ion source that is composed of electrodes which aretransparent to the ions being studied. Ions generated are acceleratedout of this ion source along the axis of the analyzer via electricpotentials on two or three metal planar electrodes. The ions are thenreflected by a reflectron back towards the ion source. According toMamyrin, by the time the ions arrive back at the source, the sourceelectrodes are deenergized so that the ions can pass through the sourceand into the detector. However, the ion source of Mamyrin et al. is notdesigned in such a way as to be useful as a reflectron or reflectingdevice. Furthermore, Mamyrin et al. neither teach nor suggest any methodof ion analysis via multiple passes through reflecting devices.

[0033] It has been suggested by Wollnik, H., in Time-of-flight MassAnalyzers, Mass Spec. Rev., 1993, 12, p.109, that two reflectrons may beconfigured coaxially with respect to one another in such a way that ionscan be reflected back and forth between them. Wollnik's prior artspectrometer is depicted in FIG. 5. (see also, Wollnik et al., SpectralAnalysis Based on Bipolar Time-Domain Sampling: A Multiplex Method forTime-of-Flight Mass Spectrometry, Anal. Chem., 1992, 64, p.1601, andHerman Wollnik, UK Patent Application No. 8120809 and German Patent No.3025764). Parts of the spectrometer according to the present inventionresemble this arrangement superficially, however, as will be seen below,the present invention has some significant differences with regard toboth the apparatus and method.

[0034] In the hypothetical instrument as shown in FIG. 5, Wollniksuggests that two reflectrons 50, 52 be placed coaxially with respect toone another, that an ion source 54 be placed at one end of theinstrument, and that a detector 56 be placed at the other end. The ionsource 54 is used to generate analyte ions in a pulsed manner. The ionsare accelerated to their full analysis velocity by the ion source 54.That is, the sum of the kinetic and potential energies of the ions doesnot change significantly between the time the ions exit the ion source54 and the end of the mass analysis. Ions exit the ion source 54 fullyaccelerated and pass through the reflectron 50 (the first reflectron)immediately adjacent to the ion source 54—which at the time is at groundpotential.

[0035] After the ions have pass through reflectron 50, reflectron 50 israpidly energized to a high potential. In contrast, reflectron 52,adjacent to the detector 56, is energized before and during theanalysis. While both reflectrons 50, 52 are energized, ions arerepeatedly reflected back and forth between them (as indicated by ionpath 58). To conclude the analysis, reflectron 52 must be rapidlydeenergized to ground potential so that ions are then able to passthrough it and into the detector 56 However, Wollnik does not teach howa reflectron or similar device might be pulsed “ON” or “OFF”.

[0036] Notice again that in Wollnik's prior art spectrometer, thereflectron is not used to accelerate ions to their analysis energy.Rather, Wollnik teaches the use of the ion source 54 to accelerate theions. This is because Wollnik's reflectron 50 is inadequate foraccelerating ions to their analysis energy but is adequate only forreflecting ions.

[0037] Consider, for example, the use of a two stage reflectron as anaccelerator in a one meter spectrometer according to Wollnik's design asdepicted in FIG. 5. In such a case the image plane would be about 0.5 mfrom the reflectron. Using equation 3 of Schiag et al. (Int J MassSpectrom. Ion Process., 112, 121 (1992)) one can determine the minimumdimensions and potentials applied to the reflectron. The minimum lengthof the reflectron would be about 7 cm. The “front” stage would be about1 cm long (X_(A2)=0.01 m in Schlag's notation) and the “back” stagewould be about 6 cm long. Assuming the ions are to have a analysiskinetic energy of about 6.75 kV, and the starting position within thereflectron to be about 0.047 m from the grid separating the twoaccelerating stages (i.e. X_(A1)=0.047 m), then, the potential appliedto the grid separating the two accelerating stages would be U_(A2)˜4.59kV and the potential applied to the back of the reflectron would be7.332 kV so that the potential at the starting position would be U=6.75kV. Under such circumstances, accelerating leu-enkephalin ions from restat X_(A1)=0.047 m would result in a flight time of the ions to the imageplane of about 14.5 microseconds. More importantly, the flight timedistribution would be large—about 12 ns or more. This is by far thelargest error in the measurement and would have a substantial negativeimpact on the resolution of the spectrometer. Indeed Schlag et al.indicate that such a device is not useful as an accelerator because “theelectric field in the first stage of the [accelerator] has to be veryweak, which induces a large spread in flight times, e.g. due to the‘turn around time’ effect.”

[0038] Also note that Wollnik does not teach how the reflectron may bepulsed rapidly to high voltage from ground or vice versa. This is animportant consideration in the construction of his proposed analyzer.Assuming, for example, the flight time of ions from one reflectron tothe other is about 30 μsec, then all of the electrodes of reflectron 1must be pulsed to the appropriate high voltages in a substantiallyshorter time than this—e.g. 1 μsec. Also, all the electrodes ofreflectron 2 must be pulsed to ground in a short time frame in order toconclude the analysis. Although one might in theory control thepotential on each and every electrode of both reflectrons with its ownindividual pulser, such would prove impractical and costly.

[0039] Finally, notice in Wollnik's spectrometer of FIG. 5, that theinstrument is limited in the ion sources that might be used with theanalyzer. The only ion sources that can be used are those external tothe analyzer, that produce ions in a pulsed manner (typicallynanoseconds in duration), and produce ions that are already at theiranalysis energy when they exit the source.

[0040] The performance of the instrument is directly influenced by theduration of the ion pulses produced by the source. That is, the pulse ofions finally observed at the detector cannot be shorter in duration thanthe duration of the ion pulse produced at the source. As the massresolving power of the instrument is inversely proportional to the ionpulse duration at the detector, is it is clear that the duration of theion pulse produced at the source is of critical importance in theperformance of the instrument as a whole. Also, the signal-to-noiseratio and therefore the limit of detection of the instrument is relatedto the width of the ion pulse. Broader pulses will result in a lowersignal-to noise ratio and a lower limit of detection.

[0041] The purpose of the present invention is to provide a means andmethod for operating a device which can serve as an accelerator and areflectron in a TOF mass spectrometer and which can also be energizedand deenergized in a pulsed manner. It is a further purpose of thepresent invention to provide a means and method of operating a massspectrometer which uses said device to accept ions from a source whichis either external or internal to the analyzer and analyze them in a TOFmass analyzer wherein ions are reflected multiple times between saiddevice and one or more reflectrons for the purpose of improving the massresolution of the instrument. It is a further purpose of the presentinvention to provide a means and method of operating a mass spectrometerthe resolution of which is substantially not influenced by the durationof the ion pulse produced by the ion source and wherein ions arereflected multiple times between one or more reflecting devices for thepurpose of improving the mass resolution of the instrument.

[0042] Several references relate to the technology herein disclosed. Forexample, F. Hillenkamp, M. Karas, R. C. Beavis, B. T. Chait, Anal. Chem.63(24), 1193A(1991); Wei Hang, Pengyuan Yag, Xiaoru Wang, ChenglongYang, Yongxuan Su, and Benli Huang, Rapid Comm. Mass Spectrom. 8,590(1994); A. N. Verentchikov, W. Ens, K. G. Standing, Anal. Chem. 66,126(1994); J. H. J. Dawson, M. Guilhaus, Rapid Comm. Mass Spectrom. 3,155(1989); M. Guilhaus, J. Am. Soc. Mass SpecLrom. 5, 588(1994); E.Axelsson, L. Holmlid, Int. J. Mass Spectrom. Ion Process. 59, 231(1984);O. A. Mirgorodskaya, et al., Anal. Chem. 66, 99(1994); S. M. Michael, B.M. Chien, D. M. Lubman, Anal. Chem. 65, 2614(1993); W. C. Wiley, I. H.McLaren, Rev. Sci. Inst. 26(12), 1150(1955).

SUMMARY OF THE INVENTION

[0043] The present invention relates generally to time-of-flight massspectrometers. More specifically, the invention comprises an improvedmethod and apparatus for analyzing ions using a time-of-flight massspectrometer. In the present invention, two or more ion reflectingdevices are positioned with respect to one another such that ionsgenerated by an ion source can be reflected back and forth between them.

[0044] The first reflecting device is an ion accelerator whose functionis two-fold. First, it acts as an accelerating device and provides theinitial acceleration to ions received from the ion source. Second, theaccelerator acts as a reflecting device and reflects the ions in thesubsequent mass analysis. As discussed above and in reference to theprior art work of Mamyrin and Wollnik, the ability of the acceleratoraccording to the present invention to act both to accelerate the ionsand reflect them in the subsequent analysis is an important feature ofthe instrument according to the present invention.

[0045] The second reflecting device is a reflectron and acts only toreflect ions in such a manner that all ions of a given mass-to-chargeratio have substantially the same flight time through the analyzer.During ion analysis, ions are reflected back and forth between theaccelerator and reflectron(s) multiple times. At the end of the ionanalysis, the accelerator is rapidly deenergized so as to allow the ionsto pass through the accelerator and subsequently into a detector.Alternatively, the reflectron is rapidly deenergized so as to allow theions to pass through the reflectron and subsequently into a detector.Alternatively, the analysis may be concluded by deflecting the ions intoa detector using electrostatic deflection plates or one of thereflectrons might be rapidly deenergized so as to allow ions to passthrough it and into a detector located behind it. Importantly, theelements of the accelerator and/or reflectron(s) are energized anddeenergized in a pulsed manner via a resistor-capacitor (RC) dividerspecifically designed for this purpose.

[0046] By reflecting the analyte ions back and forth between the isaccelerator and the reflectron several times, a much longer flight pathcan be achieved in a given size spectrometer than could otherwise beachieved. Consequently, the mass resolving power of the TOF massspectrometer taught here can be substantially greater than couldotherwise be achieved in a TOF mass spectrometer of similar size.

[0047] Notice that because the present invention uses a speciallydesigned accelerator, the present invention does not require and doesnot use an ion source which generates high kinetic energy ions in apulsed manner. Rather, the present invention can employ a variety of ionsources that produce relatively low kinetic energy ions. The ion sourceaccording to the present invention may be either internal or external tothe accelerator. Also, ions can be injected into the accelerator ineither a pulsed, continuous, or semi-continuous manner. In contrast toWollnik's prior art, the performance of the present invention in termsof mass resolving power is not substantially influenced by the width ofthe ion pulse produced by the ion source. Rather, the analysis of theions is initiated when the accelerator is pulsed “ON”. That is, thepulsing of the accelerator forms the ions into a well defined ion pulse.By pulsing the accelerator “ON” for about 100 ns, the ions can be formedinto a pulse which is on the order of a 2 ns duration regardless of theduration of the ion pulse provided by the source.

[0048] Other objects, features, and characteristics of the presentinvention, as well as the methods of operation and functions of therelated elements of the structure, and the combination of parts andeconomies of manufacture, will become more apparent upon considerationof the following detailed description with reference to the accompanyingdrawings, all of which form a part of this specification.

BRIEF DESCRIPTION OF THE DRAWINGS

[0049] A further understanding of the present invention can be obtainedby reference to a preferred embodiment set forth in the illustrations ofthe accompanying drawings. Although the illustrated embodiment is merelyexemplary of systems for carrying out the present invention, both theorganization and method of operation of the invention, in general,together with further objectives and advantages thereof, may be moreeasily understood by reference to the drawings and the followingdescription. The drawings are not intended to limit the scope of thisinvention, which is set forth with particularity in the claims asappended or as subsequently amended, but merely to clarify and exemplifythe invention.

[0050] For a more complete understanding of the present invention,reference is now made to the following drawings in which:

[0051]FIG. 1 illustrates the geometry associated with a prior artaccelerator;

[0052]FIG. 2 illustrates the geometry associated with a prior artreflectron;

[0053]FIG. 3 illustrates the geometry associated with a prior artinstrument which employs the prior art accelerator of FIG. 1 and theprior art reflectron of FIG. 2;

[0054]FIG. 4 shows a prior art mass spectrometer as disclosed by Mamyrinet al.;

[0055]FIG. 5 shows a prior art mass spectrometer as disclosed byWollnik;

[0056]FIG. 6 shows a block diagram of a preferred embodiment of a massspectrometer according to the present invention;

[0057]FIG. 7 shows a diagram of a preferred embodiment of an orthogonalinterface according to the present invention;

[0058]FIG. 8 shows a plot of the capacitance of the capacitors of the RCdivider used in conjunction with the accelerator as a function of theirorder in the accelerator in accordance with the present invention;

[0059]FIG. 9 shows a spectrum obtained via the survey method ofoperation of a mass spectrometer according to a preferred embodiment ofthe present invention;

[0060]FIG. 10 shows a plot of four mass spectra of leu-enkephalinobtained using amass spectrometer instrument in accordance with apreferred embodiment of the present invention;

[0061]FIG. 11 illustrates a timing diagram showing the sequence ofevents which may occur in an example ion analysis using a massspectrometer in accordance with a preferred embodiment of the presentinvention;

[0062]FIG. 12 depicts the geometry associated with a mass spectrometerin accordance with a preferred embodiment of the present invention;

[0063]FIG. 13 shows a plot of the potential V; applied to the reflectronof a preferred embodiment of the present invention as a function of thenumber of reflections (n);

[0064]FIG. 14 shows a plot of the flight time of leu-enkephalin ions asa function of the number of passes made through a mass spectrometerinstrument in accordance with a preferred embodiment of the presentinvention;

[0065]FIG. 15 shows the observed upper and lower mass limits of themass-to-charge (m/q) of ions plotted as a function of the time theaccelerator is pulsed to ground potential, assuming n=2;

[0066]FIG. 16 shows a plot of the optimum resolution that can beobtained as a function of n using a mass spectrometer in accordance witha preferred embodiment of the present invention;

[0067]FIG. 17 shows an alternate embodiment of a time-of-flight massspectrometer according to the present invention wherein the acceleratoris not pulsed but the reflectron is pulsed on and off to allow for theanalysis and detection of ions, respectively;

[0068]FIG. 18 shows a diagram of an alternate embodiment of anorthogonal interface according to the present invention wherein theaccelerator is used as a two stage accelerator;

[0069]FIG. 19 shows a diagram of an alternate embodiment of theaccelerator according to the present invention wherein the capacitors ofthe RC network are formed from the electrodes of the accelerator;

[0070]FIG. 20 shows an alternate embodiment of a mass spectrometeraccording to the present invention wherein neither the accelerator northe reflectron are pulsed and ions are deflected at the end of ananalysis by a deflecting device onto a trajectory leading to a detector;and

[0071]FIG. 21 shows an alternate embodiment of a mass spectrometeraccording to the present invention wherein three reflecting devices areused and ions are reflected back and forth between them along a “v”shaped path.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0072] As required, a detailed illustrative embodiment of the presentinvention is disclosed herein. However, techniques, systems andoperating structures in accordance with the present invention may beembodied in a wide variety of forms and modes, some of which may bequite different from those in the disclosed embodiment. Consequently,the specific structural and functional details disclosed herein aremerely representative, yet in that regard, they are deemed to afford thebest embodiment for purposes of disclosure and to provide a basis forthe claims herein which define the scope of the present invention.

[0073] The following presents a detailed description of a preferredembodiment of the present invention, as well as some alternateembodiments of the invention. As discussed above, the present inventionrelates generally to the mass spectroscopic analysis of chemical samplesand more particularly to time-of-flight mass spectrometry. Specifically,an apparatus and method are described for the analysis of ionizedspecies in a spectrometer comprising two or more reflecting devices suchthat ions can be reflected back and forth a plurality of timestherebetween. Reference is herein made to the figures, wherein thenumerals representing particular parts are consistently used throughoutthe figures and accompanying discussion

[0074] With reference first to FIG. 6, shown is a block diagramdepiction of a mass spectrometer analyzer 60 according to a preferredembodiment of the present invention. The analyzer 60 shown comprises an“orthogonal interface” 62, a vacuum chamber 67, and a single stagereflectron 68 all oriented coaxially with one another. The orthogonalinterface 62 accepts ions from an external ion source 61, for example anelectrospray ionization (ESI) source, and accelerates them toward areflectron 68 and ultimately detects them via detector 63 or 69. Avariety of other sources might be used including sources that operateentirely under vacuum and those in which ions are formed at elevatedpressures. These include, for example, a matrix assisted laserdesorption ionization source, a chemical ionization source, an electronionization source, an atmospheric pressure chemical ionization source,or a secondary ionization source. Also, the ion source might include anion as described by C. M. Whitehouse and Erol Culcicek in U.S. Pat. No.5,652,427 and ion storage as described by T. Dresch in U.S. Pat. No.5,689,111.

[0075] Furthermore, rather than accept ions directly from an ion source61, the orthogonal interface 62 might instead accept ions from someother device interposed between the ion source 61 and the orthogonalinterface 62. For example, one might analyze ions from an ion sourceusing a quadrupole analyzer before injecting them into the orthogonalinterface. Further, one might dissociate ions from the ion source anduse the TOF analyzer to analyze the products of the dissociation.

[0076] The preferred embodiment of the orthogonal interface 62 comprisesa single stage accelerator 64 and a detector 63 positioned behind theaccelerator 64. In addition, a preferred embodiment of the massspectrometer according to the present invention further comprises a“multideflector” 65 according to U.S. Pat. No. 5,696,375.

[0077] In the preferred embodiment, ions enter the accelerator 64 in adirection orthogonal to the axis of the analyzer 60 while theaccelerator 64 is being held at ground potential. The accelerator 64 isthen pulsed to a high voltage to accelerate the ions along the axis ofthe analyzer and in the direction of the reflectron 68. However, afteracceleration, the ions still have their initial kinetic energy in theorthogonal direction. A multideflector 65 is used to deflect the ionsonto a trajectory which is truly parallel to the axis of the analyzer60. The ions then drift through the field free region 67 of the analyzer60 until encountering the reflectron 68. In the reflectron 68, the ionsare reflected back toward the orthogonal interface 62. The potential onthe accelerator 64 is again held at ground by the time the ions arriveback at the orthogonal interface 62. The ions are thus able to passunhindered through the accelerator 64 and into the detector 63.

[0078] Not shown in FIG. 6 are the electronics required to control andprovide power to the various-elements in the spectrometer. Specifically,high voltage power supplies and pulsers, timing pursers, an oscilloscopeor other similar device, and computers—all of which are commonly knownin the industry and commercially available.

[0079] The preferred embodiment according to the present invention issimilar to Mamyrin's prior art spectrometer shown in FIG. 4 anddiscussed herein with regards to FIG. 4. However, there are someimportant differences between the prior art spectrometer of Mamyrin andthe present invention. The Mamyrin spectrometer, for example, uses anion source which is integral to the mass analyzer. In contrast, the ionsource according to the preferred embodiment of the present invention isexternal to the mass analyzer. The ion source of the present inventionforms ions into a low energy beam and injects them into the analyzer ina direction which is orthogonal to the direction in which the TOF massanalysis is to occur. For the purpose of this discussion, “low energy”means that the ions have a kinetic energy which is small in comparisonto the kinetic energy the ions will have once they have been acceleratedby the accelerator. For example, the ions produced by the ion sourcemight initially have kinetic energies of 10 eV/ion, but once the ionsare accelerated by the ion source they might have kinetic energies of10,000 eV/ion. The use of external ion sources in the present inventionrepresents an enhancement in flexibility over prior art.

[0080] The ion accelerator according to the present invention is alsosubstantially different from Mamyrin's accelerating electrodes. Mamyrinused at most three accelerating electrodes. In FIG. 4 these are labeled5, 6, and 13. A pulsed electrical potential is applied only to electrode5. The remaining two are held at a DC potential. Mamyrin describes ionformation as occurring between electrodes 5 and 6. Therefore, Mamyrinteaches that the distance “between the electrodes . . . must be as smallas possible but still 3 to 5 times the width of the ion formationregion.” Clearly then, the distribution of starting positions of theions within the source region would be between ⅕ and ⅓ of the gapbetween electrodes 5 and 6. When the electrical potential on electrode 5is pulsed, the potential energy of the ions between electrodes 5 and 6are abruptly changed. The potential energy of a given ion at this pointin time is directly dependent on its position with respect to electrodes5 and 6. Ions closer to electrode 5 at the time of the pulse will have ahigher potential energy than those further away from electrode 5.

[0081] Thus, the broad spatial distribution taught by Mamyrin leads to abroad potential energy distribution and ultimately a broad kineticenergy distribution in the ions under analysis. As discussed withreference to FIGS. 2 and 3, it is possible to use a reflectron totemporally focus the ions. However, the distribution of kinetic energiesthat can be focused by a given type of reflectron is limited. Asdiscussed above with regard to FIG. 2, a single stage reflectron canfocus ions over a kinetic energy range of less than 5% of the nominalion energy. For example, to focus ions in the preferred embodiment ofthe present invention to a flight time distribution of about 1 ns fromplane P₂ to plane P₃, it is necessary that the kinetic energy of theions vary by no more than 150 eV out of a nominal 7,000 eV kineticenergy beam.

[0082] The accelerator according to the present invention is thereforeintended to be as long as possible while considering other factors suchas electric field strength and the initial ion energy distribution. Bymaking the accelerator as long a possible, the final kinetic energydistribution of the ions due to their initial spatial distribution willbe minimized. However, as discussed by Wiley and Maclaren, because theions is initially have a small amount of kinetic energy directed alongthe axis of TOF analyzer, the ions will have a distribution of flighttimes to the image plane P₂. The reflectron cannot compensate for thisdistribution of flight times. To reduce this effect, one must reduce theion's initial energy distribution or increase the field strength in theaccelerator. The lower the accelerating field strength, the moresignificant the distribution in ion flight times will be.

[0083] To maintain the accelerating field strength and increase thelength of the accelerator, the potential applied to the acceleratorshould be as high as possible. For example, in the preferred embodimentof the accelerator according to the present invention, the length of theaccelerator is about 60 mm whereas the distribution of initial positionsof the ions around the object plane P₁ is about 1 mm. This results in anenergy distribution of about {fraction (1/60)}=1.7% of the nominal ionkinetic energy. To reduce the effect of the initial ion kinetic energy,the potential applied to the accelerator is as high as practical, forexample, 10 kV.

[0084] Having a long accelerator has the additional advantage that itreduces the influence of the “fringe” field at the entrance ofaccelerator on the trajectory of the ion beam. As in the Mamyrin priorart, the preferred embodiment of the accelerator according to thepresent invention is bounded at either end by planar conducting mesh.Ions are accelerated from their starting positions at plane P₁ to theend of the accelerator where they pass through the conducting mesh andinto the field free drift region of the spectrometer. Ideally, on theaccelerator side of the conducting mesh is the acceleratingelectrostatic field and on the opposite side is a field free region.However, because the mesh is essentially a grid work of metal wires, theaccelerating electrostatic field can to some degree penetrate throughthe small holes of the mesh. Near the holes in the mesh, the field isthus distorted and the distorted accelerating field can deflect the ionsfrom the axis of the analyzer and onto a trajectory where the flighttime will be distorted or the ion will be lost. Such deflection isdependent on the kinetic energy of the ion when it encounters the meshand on the field strength in the accelerator. By having a longaccelerator the field strength can be maintained while increasing theion's kinetic energy at the mesh. Thus, having a long acceleratorreduces the influence of the mesh on the trajectory of the ions.

[0085] Turning next to FIG. 7, shown is a cross-sectional view of thepreferred embodiment of the orthogonal interface 62. The vacuum sheath70 is preferably an electrically conducting tube. It serves to separatethe vacuum of the orthogonal interface 62 from that of the ion source61. Also, the vacuum sheath 70 provides a grounded conducting surfacethat acts to electrically shield the orthogonal interface 62 and producea reproducible capacitance between ground and the accelerator electrodes78. As discussed below, the stray capacitance between the acceleratorelectrodes 78 and ground can be an important consideration in theproduction of a uniform accelerating field. Preferably, the electrodes78 of the accelerator 64 are stainless steel rings. Conducting mesh 71,76 (91% transmission, 70 lines/inch) is supported on the rings at eitherend of the accelerator 64. The gridless steel rings (electrodes 78)(eleven (11) are shown) together with the two (2) gridded rings (mesh72, 76) are spaced at regular intervals along the axis of theaccelerator 64. Adjacent electrodes are electrically connected to oneanother by capacitors and resistors 74, as depicted in FIG. 7. That is,a single resistor connects two adjacent electrodes and a singlecapacitor in parallel to the resistor also connects these twoelectrodes. The resistors used in the preferred embodiment have aresistance of about 5 Mohms, and the capacitors have a capacitance ofabout 560 pF.

[0086] Ions 71 enter the accelerator 64 near its end which is

[0087] adjacent to the detector 63 while both ends of the accelerator 64are held at ground potential. Once ions are in the accelerator 64, oneend of the accelerator 64 is pulsed to a high voltage. The potentialapplied to the end of the accelerator is divided by the capacitors andresistors 74 so that each electrode 78 of the accelerator 64 has apotential applied to it according to its position within the accelerator64. That is, if the electrode nearest the multideflector 65 is held atground potential and the electrode nearest the detector 63 is pulsed to,for example, 10 kV, then the electrode adjacent to that closest thedetector 63 should be pulsed by the RC divider 74 to 10 kV*{fraction(11/12)}=9.16 kV because it is located at a position {fraction(11/12)}^(th) of the distance between the grounded electrode and thepulsed electrode. Similarly, the electrode midway between the groundedand pulsed electrodes should be pulsed through the RC network 74 to apotential of 10 kV*{fraction (6/12)}=5 kV. In this way a uniformelectrostatic field can be produced within the accelerator 64. Noticethat when the potential on the end of the accelerator 64 is brought backto ground, all the electrodes 78 of the accelerator 64 aresimultaneously brought back to ground potential by the RC divider 74.Thus, ions returning from the reflectron 68 after the accelerator 64 isbrought back to ground potential will encounter no electric field in theaccelerator 64 and will pass unhindered into the detector 63.

[0088] Care must be taken when determining the optimum capacitances ofthe capacitors used in the accelerator 64. Particularly, the “stray”capacitance between the electrodes 78 of the accelerator 64 and nearbyconductors can have a substantial influence on the division of theapplied pulsed voltage. In the preferred embodiment, for example, thecapacitance between each electrode 78 and the vacuum sheath 70 is about0.7 pF. As a result, if the capacitance of all the capacitors, C₀-C₁₁were to be a fixed value, for example 530 pF, the field gradientproduced would be non-uniform having a high field gradient near thepulsed end of the accelerator 64 and a lower field gradient near thegrounded end of the accelerator 64. Once the capacitance between theelectrodes 78 and ground (i.e. the vacuum sheath) is known, it is atrivial matter to calculate from elementary physics the correctcapacitance for each of the capacitors so that a uniform field isproduced. For example, if it is assumed that each of the electrodes 78of the accelerator 64 has the same capacitance with the vacuum sheath70, C_(v), then it is easy to show that the capacitance of thecapacitors required to produce a homogeneous field in the accelerator 64is given by: $\begin{matrix}{C_{j} = {C_{0} + {\sum\limits_{i = 1}^{j}{i\quad C_{v}}}}} & (5)\end{matrix}$

[0089] Assuming C_(v)=1 pF, and C₀=530 pF, one obtains capacitances asplotted in FIG. 8.

[0090] In fact it is possible by assuming an appropriate value for C_(v)to determine the capacitance values necessary to produce a field thatnot only accelerates the ions but also provides limited lateralfocusing. The actual capacitance between each electrode 78 and thevacuum sheath 70 is about 0.7 pF. By assuming a value of 1 pF for C_(v)instead of 0.7 pF, the calculated capacitances will be greater thannecessary to produce a uniform field. Instead the field will be weakernear the detector 63 end of the accelerator 64 and stronger near themultideflector 65 end. This results in an electric field component whichacts normal to the axis of the accelerator 64 and deflects the ionstoward the axis of the accelerator 64. Having a small amount of suchlateral focusing does not significantly influence the flight time of theions but can improve the transmission efficiency of ions to thereflectron 68 and back.

[0091] To produce a uniform field that can be turned “ON” and “OFF” in apulsed manner using prior art methods and only one high voltage pulserwould require that one use two and only two electrodes in an arrangementsimilar to that of Mamyrin as discussed in reference to FIG. 4. In anaccelerator similar to Mamyrin's prior art accelerator only oneelectrode is pulsed whereas the other is held at a fixed potential. Inorder to produce a uniform accelerating field the spatial extent of theis accelerating electrodes normal to the axis of the analyzer is largecompared to the gap between the electrodes. To increase the length ofthe accelerator and maintain a uniform field, the extent of theaccelerating electrodes normal to the analyzer axis must be increasedFor example, if the length of the accelerator was to be increased to 60mm the extent of the electrodes normal to the analyzer axis would haveto be increased to about 200 mm in diameter. Thus, the accelerator 64according to the present invention has the advantage over prior art thatit can be used to produce a uniform, switchable accelerating field usinga single pulser wherein the spatial extent of the accelerating field inthe direction of acceleration is large in comparison to the initialspatial extent of the ion beam and wherein the extent of the accelerator64 normal to the axis of the accelerator 64 is not large in comparisonto its extent along its axis.

[0092] While FIG. 7 depicts a high voltage pulse being applied to theend of the accelerator 64 adjacent to the detector 63 and ground asbeing applied to the end of the accelerator 64 adjacent to themultideflector 65, it should be understood that any combination of DCand pulse voltages could be applied to either end of the accelerator 64.For example, by holding the detector 63 end of the accelerator 64 atground and pulsing the multideflector 65 end of the accelerator 64 to ahigh voltage, ions starting near the detector 63 end of the accelerator64 are accelerated directly into the detector 63. Or by applying alesser pulsed voltage to the detector 63 end of the accelerator 64 and agreater pulsed voltage to the multideflector 65 end of the accelerator64, one can accelerate ions through two acceleration stages—one withinthe accelerator 64 itself and a second between the end of theaccelerator 64 and the detector 63. It is possible thus to space focusthe ions in the manner taught by Wiley and Maclaren. Because in thismode of operation, ions are accelerated directly into the detector 63,it is possible to achieve a higher sensitivity than in reflectron modewhere ions travel to the reflectron 68 and back. An example of such a“survey spectrum” is shown in FIG. 9 for a sample of leu-enkephalin.

[0093] In “reflectron mode”, a high voltage pulse is applied to theaccelerator 64 as shown in FIG. 7 so that ions are accelerated towardthe reflectron 68. When operating the instrument in a manner similar tothat taught by Mamyrin, one would lower the potential on the accelerator64 to ground after the ions of interest have been fully accelerated andbefore ions of interest have returned from the reflectron 68. However,by maintaining the potential on the accelerator 64 for a longer periodof time, ions returning from the reflectron 68 can be reflected by theaccelerator 64 back toward the reflectron 68. In this manner, the ionsmay be reflected multiple times between the reflectron 68 and theaccelerator 64. To complete the analysis, the accelerator 64 isdeenergized so that ions may pass through it and into the ion detector63. Alternatively, the reflectron 68 may be deenergized so that ions maypass through it and into the ion detector 69. Assuming the appropriatevoltages and timing are used, one can obtain higher resolution spectraby having multiple reflections rather than the single reflection by thereflectron 68.

[0094] Example spectra of leu-enkephalin obtained in reflectron mode areshown in FIG. 10. The first spectrum 101 of leu-enkephalin was obtainedwith a single reflection of the ions by the reflectron. The secondspectrum 102 was obtained via a reflection of the ions by the reflectronfollowed by a reflection by the accelerator and then a second reflectionby the reflectron.

[0095] The timing of this experiment is illustrated in FIG. 11. In thefirst trace 111, “Source Ion Pulse”, a pulse of ions is generated by theion source 61. The ions travel from the source 61 to the orthogonalinterface 62 and into the accelerator 64. Then, as shown in the secondtrace 112, the “Accelerator High Voltage Pulse” trace, the accelerator64 is pulsed to and maintained at a high voltage for some predeterminedperiod of time—in this example 130 us. As indicated in the second trace112, pulsing the accelerator 64 to a high voltage accelerates the ionstoward the reflectron 68 and, in effect, forms them into ion packets. Atsome later time the ions arrive at the reflectron 68 and are reflected.The reflected ions drift back through the flight tube 67 to theaccelerator 64 where they are reflected back to toward the reflectron68. The ions again travel through the flight tube 67 to the reflectron68 where they are reflected back toward the accelerator 64. At some timeafter the ions of interest have been reflected by the accelerator 64 andbefore they arrive back at accelerator 64 a second time, the acceleratorpotential is pulsed to ground. The ions of interest then pass throughthe accelerator 64 and into the detector 63.

[0096] The second passage of the ions through the flight tube 67effectively doubles the length of the analyzer 60 and thereby improvesthe mass resolving power of the instrument in this case from 10,000 in asingle pass to 17,000. The third spectrum 103 and fourth spectrum 104 ofFIG. 10 were obtained with three and four reflections of the ions by thereflectron 68, respectively (i.e., the ions made three and four completepasses through the analyzer 60, respectively). These spectra showimprovements in resolution to 20,000 and 23,000 respectively.

[0097] In order to successfully analyze ions as outlined above, theproper accelerator and reflectron potentials and the proper timing ofthe accelerator 64 pulse must be used. Two methods for the determinationof the appropriate potentials and timing according to the presentinvention are therefore outlined below.

[0098]FIG. 12 depicted is the geometry of a preferred embodiment of theanalyzer 60 (see FIG. 6) according to the present invention. For thepresent discussion, it is assumed that the end of the ion accelerator 64closest to the reflectron 68 and the end of the reflectron 68 closest tothe accelerator 64 are held at ground potential. Potentials V₁ and V₂are applied to the opposite ends of the accelerator 64 and reflectron68, respectively. Potential V₁ is a pulsed potential as described withrespect to FIG. 7 whereas potential V₂ is a constant DC potential. Theaccelerator 64 is a single stage accelerator as described with referenceto FIG. 7. The reflectron 68 is preferably a single stage reflectron asdescribed with reference to FIG. 2. The electrodes of the reflectron 68are preferably stainless steel rings. As with the accelerator 64, theseelectrodes are distributed at regular intervals along the axis of thereflectron 68. The reflectron 68 is bound at either end by conductingmesh.

[0099] The potential V₁ should simply be as large as possible. The onlylimitations on V₁ would be the practical limitations due to arcing attoo high a potential or digitization rates in the flight time recordingdevice. If, for example, V₁ were too high then the resultant massspectral peaks might be too narrow for the digitizer (oscilloscope) toproperly record. In such a case, the resolution of the spectrometerwould be limited by the digitizer. Improved resolution in such a casewould be obtained at a lower voltage. In practice, the operator shouldselect the highest practical potential for V₁.

[0100] To determine the potential for V₂, one must consider the geometryof the accelerator 64 and reflectron 68, the potential previouslyselected for V₁, and the starting position of the ions. The specificcase discussed here is for a single stage reflectron and a single stageaccelerator, however, to determine V₂ for any combination of singleand/or multistage devices, one must set the distance traveled by the ionin the field free region of the spectrometer equal to the sum of the“focal lengths” of the accelerator and reflectron times the number oftimes the ion travels through them and then solve for V₂. Here “focallength” refers to the distance, outside of the reflecting device,between an initial object/image plane and that image plane formed by theaccelerator or reflectron.

[0101] From equation 4 and FIG. 12, the effective focal length, F_(r),is for a preferred embodiment for a reflectron is given by:

F_(r)=4ns₂  (6)

[0102] where n is the number of times the ions have passed through thereflectron. From the discussion with regard to FIG. 1, the focal length,F_(o), for a preferred embodiment for an accelerator as it is initiallyaccelerating ions from their starting positions is given by:

F_(o)=2s₁  (7)

[0103] Finally, the focal length, F_(a), for the preferred embodimentfor an accelerator as it reflects ions that are returning from theaccelerator is given by:

F _(a)=4(n−1)s ₁  (8)

[0104] The factor “n−1” in equation 8 arises from the fact that ionswhich have been reflected by the reflectron n times and then detectedhave only been reflected by the accelerator n−1 times. Clearly from FIG.12, the distance, D_(ff), traveled by the ions in the field free regionof the analyzer is, in the preferred embodiment, given by:

D _(ff)=2nd₃ +d ₁ +d ₄  (9)

[0105] Setting the total distance traveled in the field free region ofthe analyzer equal to the total effective focal length gives thecondition for optimizing the mass resolution of the analyzer:

D _(ff=F) _(o) +F _(a) +F _(r)  (10)

and, 2nd₃ +d ₁ +d ₄=2s₁+(n−1)4s₁+4ns₂  (11)

[0106] The sum of the potential and kinetic energy of the ions duringthe analysis is fixed at the time the high voltage pulse is applied tothe accelerator. The total energy of the ion during the analysis isgiven by:

ε=qV ₁ s ₁ /d ₁  (12)

[0107] where q is the charge on the ion The total energy of the ion isalso related to s₂ by:

ε=qV ₂ s ₂ /d ₂  (13)

[0108] By these two equations, one finds:

S ₂ =s ₁ d ₂ V ₁ /d ₁ V ₂  (14)

[0109] Substituting this into equation 11 and solving for V₂ oneobtains:

V ₂=4nV ₁ s ₁(d ₂ /d ₁)/(2nd₃ +d ₄ +d ₁−4s₁(n+½))  (15)

[0110] Assuming d₁=0.06 m, d₂=0.47 m, d₃=0.83 m, d₄=0.041 m, s₁=0.0575m, and V₁=6,800 V, one obtains the plot of V₂ vs n shown in FIG. 13.

[0111] To summarize, the method described in detail above to determinethe appropriate potential V₂ comprises the following steps:

[0112] 1) selecting a potential for V₁ based on practical limitations;

[0113] 2) determining a first equation describing the total distancetraveled by the ions in the field free regions of the spectrometer as afunction of n;

[0114] 3) determining a second equation describing the total effectivefocal length of the accelerating and reflecting devices as a function ofn;

[0115] 4) deriving a third equation by setting the first equation equalto the second equation;

[0116] 5) determining a fourth equation(or set of equations) relatingthe potential(s) applied to the reflectron to those applied to the otherreflecting and accelerating devices in the spectrometer;

[0117] 6) deriving a fifth equation (or set of equations) bysubstituting said forth equation(s) for the reflectron potential(s) insaid third equation; and

[0118] 7) solving said fifth equation (or set of equations) for thepotential(s) applied to the reflectron as a function of n.

[0119] This method as generally defined in steps 1 through 7 isdemonstrated above for a specific instance but can be applied toanalyzers having any combination of single stage or multistageaccelerating or reflecting devices. Wiley and Maclaren, for example,have derived an equation for determining the focal length of a two stageaccelerator. Schlag et. al. have described a similar equation fordetermining the focal length of a two stage reflectron. Using suchequations in conjunction with steps 1 through 7 would give a solutionvalid for an analyzer with a two stage accelerator and a two stagereflectron.

[0120] Alternatively, or in conjunction with steps 1 through 7 above,the optimum value for V₂ might be determined by calculating a value forV₂, setting the potential on the reflectron in successive experiments toseveral values near the calculated potential, and then evaluating theresolution of the spectra obtained to determine a new value for V₂. Byrepeating such measurements many times while readjusting V₂ alwaystowards that value which provides the best resolution of any givenseries of measurements, the value of V₂ which provides the optimumresolution can be determined.

[0121] Having all the potentials and geometry set, it is a simple matterto determine the flight time of a given m/q ion through the analyzer.The flight time of an ion would be the sum of the time the ion spends inthe accelerator, t_(a), the reflectron, t_(r), and the field free,t_(ff), regions of the spectrometer. The flight time of the ion in thefield free region of the preferred embodiment analyzer is given by:

t _(ff)=(2nd₃ +d ₁ +d ₄)(md ₁/2s ₁ qV ₁)^(1/2)  (16)

[0122] The flight time of the ion in the accelerator according to thepreferred embodiment is given by:

t _(a)=(2n−1)(2s ₁ d ₁ m/qV ₁)^(1/2)  (17)

[0123] Finally, the flight time of the ion in the reflectron accordingto the preferred is given by:

t _(r)=(2nd₂ /V ₂)(2s ₁ V ₁ m/qd ₁)^(1/2)  (18)

[0124] Combining equations 16, 17, and 18 yields the total flight time(tof):

tof=2n(m/q)^(1/2) [d ₃(d _(1/2) s ₁ V ₁)^(1/2)+(2s ₁ d ₁ /V ₁)^(1/2)+(d₂ /V ₂)(2s ₁ V ₁ /d ₁)¹2]+(m/q)^(1/2)[(d ₁ +d ₄)(d ₁/2s ₁ V ₁)^(1/2)−(2s₁ d ₁ /V ₁)^(1/2)]  (19)

[0125] With the assumptions already given above and the assumption thatthe m/q of the ion is 556, the total flight time is plotted as afunction of n in FIG. 14. Note also the flight time of leu-enkephalin asobserved in the spectra of FIG. 10 are plotted in FIG. 14. Theexperimental and observed values agree to within the error of themeasurement of the geometry, and potentials of the instrument.

[0126] Given a fixed geometry and fixed potentials, equation 19 can bereduced to:

tof=(m/q)^(1/2)(an+b)  (20)

[0127] where a and b are constants. Also, for a fixed value of n,equation 20 can be reduced to:

tof=(m/q)^(1/2) A  (21)

[0128] where A is a constant. Thus, it is possible to calibrate theinstrument based on experimental results rather than from measurementsof the geometry of the analyzer and the potential applied to theaccelerator. To “calibrate the instrument” in the sense given here meansto establish a relationship between tof and m/q. Using equation 20 andmeasurements of the flight times of two different m/q ions through theinstrument or the flight time of the same m/q ions with two differentvalues of n, one can solve for a and b This solution, however, assumesthat the value of V₂ is varied with n according to equation 20. Thus,the error in the resultant calibration is dependent on the error insetting V₂ to the theoretical value.

[0129] In contrast, the instrument can be calibrated using equation 21and the flight time measurement of at least one known m/q ion. In such acase there is no longer a dependence on the setting of V₂. In practicethe measured flight time is typically offset by a constant value due to,for example, delays built into the pulsing circuits and digitizer. Thus,equation 21 would become:

tof _(m)=(m/q)^(1/2) A+B  (22)

[0130] where tof_(m) is the measured flight time and B is a constant.So, to calibrate the instrument in practice, two measurements must beused with equation 22 to experimentally calibrate the instrument.

[0131] As discussed above and in reference to FIG. 12, the accelerator64 is pulsed to a high voltage to initialize the TOF mass analysis. Theanalysis is concluded and the ions are allowed to pass through theaccelerator 64 to the detector 63 by pulsing the accelerator 64 back toground potential. To relate the ion m/q range of interest to the timethat the accelerator 64 is brought back to ground potential, one mustdetermine the m/q of the ions that would, if present, be returning fromthe reflectron 68 and entering the accelerator 64 at the time it ispulsed to ground. These would be the lowest m/q ions measurable in thegiven analysis. Similarly, the m/q of the ions that would, if present,be exiting the accelerator 64 in the direction of the reflectron 68 mustbe determined. These would be the highest m/q ions that could bemeasured in the given analysis.

[0132] To obtain the m/q of the lowest m/q ions measurable in a givenanalysis, one need only modify equation 19 so that it does not included₁ and d₄ in the calculation of the flight time of the ion in the fieldfree drift region. Equation 19 then becomes:

t _(off) =<tof=2n(m/q)^(1/2) [d ₃(d ₁/2s ₁ V ₁)^(1/2)+(2s ₁ d ₁ /V₁)^(1/2)+(d ₂ /V ₂)(2s ₁ V ₁ /d ₁)^(1/2)]−(m/q)^(1/2)(2s ₁ d ₁ /V₁)^(1/2)  (23)

[0133] Solving this for m/q yields:

m/q>=t _(off) ²/[2nd₃(d ₁/2s ₁ V ₁)^(1/2)+(2nd₂ V ₂)(2s ₁ V ₁ /d₁)^(1/2)+(2n−1)(2s ₁ d ₁ /V ₁)^(1/2)]²  (24)

[0134] To determine the high m/q limit to the m/q range equation 19 ismodified so that for a given value of n, the flight time of the ion tothe reflectron 68 and back only n−1 times is calculated and summed withthe time required for the ion to be initially accelerated by theaccelerator 64. This gives:

t _(off) >=tof=2(n−1)(m/q)^(1/2) [d ₃(d ₁/2s ₁ V ₁)^(1/2)+(2s ₁ d ₁ /V₁)^(1/2)+(d ₂ /V ₂)(2s ₁V₁ /d ₁)^(1/2)]+(m/q)^(1/2)(2s ₁ d ₁ /V₁)^(1/2)  (25)

[0135] Solving for m/q one obtains:

m/q<=t _(off) ²/{2(n−1)[d ₃(d/2s ₁ V ₁)^(1/2)+(2s ₁ d ₁ /V ₁)^(1/2)+(d ₂/V ₂)(2s ₁ V ₁ /d ₁)^(1/2)]+(2s ₁ d ₁ /V ₁)^(1/2)}²  (26)

[0136] For a given value of n, equations 24 and 26 give the bounds onthe range of m/q ions which can be detected with a given t_(off) time.As an example, FIG. 15 shows a plot of the m/q range as a function oft_(off) for n=2 assuming the conditions discussed above. Given a desiredm/q range, one can use equations 23 and 25 to determine the t_(off) timethat provides the best coverage of that m/q range for a given value ofn.

[0137] To summarize, the method described in detail above to determinethe appropriate t_(off) for a desired m/q range and value of n comprisesthe following steps of:

[0138] 1) determining a first equation, in accordance with the methoddescribed above, which relates the value of the potential(s) on thereflectron to n;

[0139] 2) deriving a second equation based on the first equation andother fixed parameters in the instrument which can be used to determinethe flight time of ions of the minimum desired m/q from their startingpositions, n−1 times reflected through the accelerator, and n timesreflected through the reflectron such that said ions are entering theaccelerator for the n^(th) time at the end of the determined flighttime;

[0140] 3) using said second equation to determine the maximum time atwhich the accelerator may be pulsed to ground potential given n and aminimum m/q;

[0141] 4) deriving a third equation based on the first equation andother fixed parameters in the instrument which can be used to determinethe flight time of ions of the maximum desired m/q from their startingpositions, n−1 times reflected through the reflectron, and n−1 timesreflected through the accelerator such that said ions would be exitingthe accelerator toward the reflectron for the n^(th) time at the end ofthe determined flight time; and

[0142] 5) using said third equation to determine the maximum time atwhich the accelerator may be pulsed to ground potential given n and amaximum m/q.

[0143] This method as generally defined in steps 1 through 5 isdemonstrated above for a specific instance but can be applied toanalyzers having any combination of single stage or multistageaccelerating or reflecting devices.

[0144] As when considering the instrument calibration above, it ispossible also to relate the m/q range to t_(off) experimentally.Assuming a given value for V₁, equations 23 and 25 can be reduced to:

(m/q)_(max)*(a ₁ n+b ₁)² <=t _(off) ²<=(m/q)_(min)*(a ₂ n+b ₂)²  (27)

[0145] where a₁, b₁, a₂, and b₂ are unknown constants, and (m/q)_(max)and (m/q)_(min) are the maximum and minimum m/q ions, respectively, inthe analyzed range. A minimum of four experimental measurements must bemade and used with equation 27 to solve for the unknown constants. Themeasured values would not be of ion flight times but rather what theupper and lower cutoffs in m/q are under given circumstances. Two uppercutoff measurements would be made to determine a₁ and b₁. Two lowercutoff measurements would be made to determine a₂ and b₂.

[0146] Thus, as a second method for determining t_(off) assuming a givenn and, a desired m/q range, one may, regardless of the number of stagesin the accelerating or reflecting devices:

[0147] 1) measure the minimum m/q that can be analyzed for a given n andt_(off) for at least two different values of n or two different valuesof t_(off);

[0148] 2) measure the maximum m/q that can be analyzed for a given n andt_(off) for at least two different values of n or two different valuesof t_(off); and

[0149] 3) using the four experimental values obtained in steps one andtwo, solve equation 27 simultaneously for the constants a₁,a₂,b₁, an b₂.

[0150] The maximum theoretical resolution of the instrument according tothe present invention is mainly dependent on focusing ability of thereflecting devices. The resolution of an instrument as measured at agiven mass spectral peak is defined to be the m/q of the peak divided bythe width of the peak at half its maximum intensity. The resolution ofTOF instruments is always less than infinity because of aberrations inthe flight times of the ions. If one considers an analysis in which theions make only one passage through the analyzer (i.e. n=1), then flighttime errors associated with the initial acceleration of the ions, theinitial deflection of the ions by the multideflector, the detection ofthe ions, the digitization of ion signals, and the inability of thereflectron to perfectly temporally focus over the distribution of ionenergies present all contribute significantly to the distribution offlight times observed in resultant mass spectra. The distribution in ionflight times for a given m/q ion, err, may be expressed as:

err=(x ²+(ny)²)^(1/2)  (28)

[0151] where y is the error associated with the reflectron and x is theflight time error associated with the rest of the instrument. Theresolution of the instrument is then given by:

R=tof/2err  (29)

[0152] Considering the results of FIG. 10 for leu-enkephalin, if oneassumes that the imperfect focusing of the reflectron results in, y, a1.3 ns distribution in ion flight times, and all other instrument errorscombined result in, x, a 3.3 ns distribution in ion flight times, thenequation 29 can be plotted as a function of n as shown in FIG. 16. Aftermany passes, the resolution of the instrument would be given by:

R=tof/2err˜(m/q)^(1/2) an/2ny=(m/q)^(1/2) a/2y  (30)

[0153] Thus, the resolution of the instrument has an upper limit whichis primarily dependent on the focusing ability of the reflectron. Forthe leu-enkephalin data of FIG. 10; this upper limit would he aboutR˜74,000 ns/2*1.3 ns=28,400. It is well known that multistagereflectrons can be used to focus ions over a broader distribution ofenergies or focus ions of a given energy distribution to a greaterextent than when using single stage reflectrons. Thus, by using amultistage reflectron one may is reduce y and thereby increase the limiton the instrument's resolution.

[0154] In the above discussions relating to the preferred embodiment ofthe invention, it was assumed that the reflectron 68 was held at a fixedpotential whereas the accelerator 64 was pulsed to high voltage and backto ground. However, it should be clear that the same structure ofelectrodes 78, mesh 72, 76, and RC network 74 described for theaccelerator 64 can be used in substantially the same manner to produceand operate the reflectron 68 in a pulsed manner. Thus, one could inalternate embodiments of the mass spectrometer pulse either or both thereflectron 68 and accelerator 64.

[0155] Therefore, FIG. 17, for example, shows an alternate embodiment ofthe TOF mass spectrometer (analyzer 171) according to the presentinvention wherein the accelerator 172 may or may not be pulsed but thereflectron 178 is pulsed “ON” and “OFF” to allow for the analysis anddetection of ions respectively. A pulse of ions is produced within theaccelerator 172 by, for example, laser 176 (laser ionization). Theaccelerator potential may be pulsed some time after the ions are formedor the ions may be immediately accelerated by the accelerator 172 alongthe axis of the analyzer 171 toward the reflectron 178. At the beginningof the analysis, the reflectron 178 is energized. Thus, ions reachingthe reflectron 178 are reflected back in the direction of theaccelerator 172. The ions are then reflected back and forth between theaccelerator 172 and reflectron 178 an indefinite number of times untilthe analysis is concluded by pulsing the reflectron 178 “OFF”. At such atime the ions are then able to pass freely through the reflectron 178and into the detector 179 adjacent to it. Note that the methodsdescribed above for determining the potentials applied to theaccelerator 172 and reflectron 178 and for determining the time at whichto pulse the potential on the reflectron 178 still apply except that theanalytical method for determining t_(off) must be modified to read:

[0156] 1) determining a first equation, in accordance with the methoddescribed above, which relates the value of the potential(s) on thereflectron to n;

[0157] 2) deriving a second equation based on the first equation andother fixed parameters in the instrument which can be used to determinethe flight time of ions of the minimum desired m/q from their startingpositions, n times reflected through the reflectron, and n timesreflected through the accelerator such that said ions would be arrivingat the reflectron for the (n+1)^(th) time at the end of the determinedflight time;

[0158] 3) using said second equation to determine the maximum time atwhich the accelerator may be pulsed to ground potential given n and aminimum m/q;

[0159] 4) deriving a third equation based on the first equation andother fixed parameters in the instrument which can be used to determinethe flight time of ions of the maximum desired m/q from their startingpositions, n times reflected through the reflectron, and n−1 timesreflected through the accelerator such that said ions would be exitingthe reflectron for the n^(th) time at the end of the determined flighttime; and

[0160] 5) using said third equation to determine the maximum time atwhich the reflectron may be pulsed to ground potential given n and amaximum m/q.

[0161] The above mentioned laser ionization may take the form of, forexample, matrix assisted laser desorption ionization (MALDI). In such acase, the accelerator electrode farthest from the reflectron would be asolid metal plate or a conducting sample probe rather than a griddedring as discussed with regard to FIG. 7. This sample plate would havedeposited on it solid sample material. The sample material under MALDIconditions would consist of sample material dissolved in a solid organicmatrix. In such a case, the accelerator might be pulsed at some timeafter the laser excites the sample material. The delay between the laserpulse and accelerator pulse and the potential applied to the acceleratormay be adjusted to perform space velocity correlation focusing andthereby improve the resolution of the instrument. Such focusing is wellknown in the literature (see for example, Reilly et al. U.S. Pat. No.5,504,326).

[0162] In the above discussions relating to the preferred embodiment ofthe invention, it was assumed that the reflectron and accelerator weresingle stage devices. However, as depicted in FIG. 18 it is possible touse these as multistage devices. As depicted in FIG. 18, one could, byadding a mesh 75 to one of the central electrodes and applying a highvoltage pulse, HV₂, to that electrode, use the accelerator 64 as a twostage device rather than the single stage device depicted in FIG. 7. Anynumber of stages of acceleration may be produced and used in this way.By a similar means, a multiple stage reflectron could be produced andoperated. Also, it should be clear that if a multistage device is used,not all stages of the device need be pulsed “ON” or “OFF” and not allstages that are pulsed need be pulsed simultaneously.

[0163] It should be understood that whereas those embodiments depictedin FIGS. 7 and 18 use fine conducting mesh to bound the accelerator 64,one or both of the terminal electrodes might instead be planar aperturedelectrodes having no such mesh. Also, the “aperture” in such a terminalelectrode might take the form of a circular hole in the otherwise solidelectrode or the aperture might take the form of a slit in the plate.

[0164] Also, it should be noted that waveforms other than simple squarewaves might be applied to input of the RC network 74 of the acceleratingor reflecting device. In the use of space velocity correlation focusingthis may be valuable in improving the resolution of the instrument overthat using a simple square wave or in improving the m/q range over whicha calibration function holds.

[0165] Turning next to FIG. 19, shown is a diagram of one possiblealternate embodiment of an accelerator 194 according to the presentinvention. As shown, the capacitors 195 of the RC network 190 are formedfrom the electrodes 198 of the accelerator 194. In this case, theconductive material of the electrodes 198 is extended toward adjacentelectrodes and a thin film of dielectric material 196 is used toelectrically insulate the electrodes from one another. The capacitancebetween two electrodes is then determined by C=ε_(o)κA/d, where ε_(o) isthe permittivity of free space, κ is the dielectric constant of thedielectric, A is the area of the electrode used to form the capacitor,and d is the distance between the surfaces of those portions of theelectrodes used to form the capacitor. The capacitors 195 of the RCnetwork 190 might be similarly used to form an alternate embodiment of areflectron.

[0166] Now, with reference to FIG. 20, shown is another alternateembodiment of the mass spectroscopic analyzer 200 according to thepresent invention, wherein the accelerator 204 or the reflectron 208 arenot necessarily pulsed. In this case, ions might be generated externalto the accelerator 204, if the accelerator 204 is pulsed “ON”, or in theaccelerator 204, in which case the accelerator 204 may be continuouslyenergized. In either case, the reflectron 208 is statically “ON” so thations may be reflected back and forth between the accelerator 204 and thereflectron 208 multiple times. To conclude the analysis ions aredeflected at the end of an analysis by a deflecting device 205,209 ontoa trajectory which ends at a detector 203. The deflecting device 205,209may, for example be a multideflector 205 or a pair of conventionaldeflection plates 209.

[0167] In yet another embodiment of a mass spectroscopic analyzer 210according to the present invention, three reflecting devices 214,216,218are used. As depicted in FIG. 21, an ion accelerator 214 is used toaccelerate the ions, and two reflectrons 216,218 are used to reflections multiple times through a v-shaped trajectory. The accelerator 214is of substantially the same design as discussed with respect to thepreferred embodiment and is operated in the same manner as theembodiments discussed above. The first reflectron 216 is designed andoperated in substantially the same manner as discussed with respect tothe other embodiments above. However, in the alternate embodimentdepicted in FIG. 21, the first reflectron 216 is a two stage reflectronas described by Frey et al. in U.S. Pat. No. 4,731,532. The dimensionsof the first reflectron 216 are sufficient that ions entering at a smallangle with respect to the axis of the first reflectron 216 can passthrough the first reflectron 216 and pass out of the first reflectron216 at a small angle with respect to the axis of the first reflectron216 but on the opposite side of the axis as it entered. Reflectrons withsuch dimensions are known for single and multiple stage reflectrons fromprior art-See, for example, Frey et al. U.S. Pat. No. 4,731,532. Frey etal. teach the use of a two stage gridless reflectron wherein ions enterand exit the reflectron at an angle of 4° with respect to the axis ofthe reflectron. The second reflectron 218 is substantially similar indesign, dimension, and operation to that of the accelerator 214. Thesecond reflectron 218 accepts ions from the first reflectron 216 and,during the analysis, reflects the ions back toward the first reflectron216.

[0168] In the alternate embodiment depicted in FIG. 21, an analysis isinitiated by pulsing the accelerator 214 to a high voltage. However,because the ions are not detected by a detector behind the accelerator214, the accelerator 214 need not be pulsed rapidly to ground. Ratherthe accelerator 214 must be at ground potential only by the time thenext analysis is to occur. Ions travel first from the accelerator 214 tothe first reflectron 216. Ions are reflected by the first reflectron 216into the second reflectron 218. While the second reflectron 218 is atits analysis potential, ions will be reflected back toward the firstreflectron 216. Ions returning to the first reflectron 216 will bereflected back toward the accelerator 214. In this manner, ions willcontinue to be reflected back and forth between the three reflectingdevices 214,216,218 so long as the potential on the second reflectron218 is held at its analysis voltage. To conclude the analysis, thepotential on the second reflectron 218 is pulsed to ground so that ionsmay pass through it and into a detector 213 behind it.

[0169] Note that although the second reflectron 218 must be pulsed toground rapidly to conclude an analysis, it need not be brought rapidlyback to the high voltage at which the analysis occurs. Rather it must beat its analysis potential only by the time the next analysis is tooccur. For convenience sake, the potential on the second reflectron 218is taken to be the same as that applied to the accelerator 214, however,it is possible to adjust the accelerator 214 and second reflectron 218potentials independently.

[0170] The methods described above for determining the potential appliedto the first reflectron and for determining the time at which to pulsethe potential on the second reflectron still applies to this embodimentexcept that the analytical method for determining t_(off) must bemodified to read:

[0171] 1) determining a first equation, in accordance with the methoddescribed above, which relates the value of the potential(s) on thefirst reflectron to n;

[0172] 2) deriving a second equation based on the first equation andother fixed parameters in the instrument which can be used to determinethe flight time of ions of the minimum desired m/q from their startingpositions, n times reflected through the second reflectron, 2n+1 timesreflected through the first reflectron, and n times reflected throughthe accelerator such that said ions would be arriving at the secondreflectron for the n^(th) time at the end of the determined flight time;

[0173] 3) using said second equation to determine the maximum time atwhich the accelerator may be pulsed to ground potential given n and aminimum m/q;

[0174] 4) deriving a third equation based on the first equation andother fixed parameters in the instrument which can be used to determinethe flight time of ions of the maximum desired m/q from their startingpositions, n times reflected through the second reflectron, 2n−1 timesreflected through the first reflectron, and n−1 times reflected throughthe accelerator such that said ions would be exiting the secondreflectron for the n^(th) time at the end of the determined flight time;and

[0175] 5) using said third equation to determine the maximum time atwhich the reflectron may be pulsed to ground potential given n and amaximum m/q.

[0176] In the case of n=0, the m/q range of the instrument would beunlimited and the second reflectron would be held always at groundpotential. Therefore if n=0 the term t_(off) does not apply.

[0177] Also, although the analytical method described above fordetermining the optimum potential to apply to the reflectron is validfor the first reflectron 216, the potentials applied to a two stagereflectron as depicted in FIG. 21 might more readily be obtained by:

[0178] 1) selecting a potential for V₁ based on practical islimitations;

[0179] 2) determining a first equation describing the total distancetraveled by the ions in the field free regions of the spectrometer as afunction of n;

[0180] 3) determining a second equation describing the total effectivefocal length of the accelerating and reflecting devices as a function ofn;

[0181] 4) deriving a third equation by setting the first equation equalto the second equation;

[0182] 5) solving said third equation to obtain a fourth equationrelating the effective focal length per pass of reflectron 1 to n;

[0183] 6) determining a fifth equation—such as equation 3a of Schiag etal. referred to above—relating the focal length of reflectron 1 and thegeometry of reflectron 1 to the optimum penetration depth, X_(A1), ofthe ion into the reflectron 1;

[0184] 7) substituting said fourth equation into said fifth equation toobtain a sixth equation relating X_(Al) to n;

[0185] 8) determining a seventh equation—such as equation 3b of Schlaget al. referred to above—relating ion energy, the length of the firststage of reflectron 1, and the focal length of reflectron 1 to thepotential applied across the first stage of reflectron 1;

[0186] 9) substituting said fourth equation into said seventh equationto obtain an eighth equation relating the potential applied across thefirst stage of reflectron 1 to n;

[0187] 10) determining a ninth equation relating the ion kinetic energy,the potential applied across the first stage of reflectron 1, the lengthof the second stage of reflectron 1 to the potential applied to the backend of reflectron 1; and

[0188] 11) substituting said eighth equation into equation 10 to obtainan equation relating the potential applied to the back of reflectron 1to n.

[0189] While the present invention has been described with reference toone or more preferred embodiments, such embodiments are merely exemplaryand are not intended to be limiting or represent an exhaustiveenumeration of all aspects of the invention. The scope of the invention,therefore, shall be defined solely by the following claims. Further, itwill be apparent to those of skill in the art that numerous changes maybe made in such details without departing from the spirit and theprinciples of the invention. It should be appreciated that theadjustable bungee cord fastening device of the present invention iscapable of being embodied in other forms without departing from itsessential characteristics.

What is claimed is:
 1. An apparatus for a time-of-flight massspectrometer, said apparatus comprising: at least one ion producingmeans for generaing ions; an ion accelerator; at least one reflectron;at least one high voltage pulse generator; at least oneresistor-capacitor network; and at least one ion detector; wherein saidaccelerator accelerates said ions generated by said ion producing means;wherein said reflectron is arranged together with the accelerator suchthat ions are reflected back and forth between said accelerator and saidreflectron while said accelerator and said reflectron are energized;wherein the capacitors of said network are arranged in parallel to theresistors of said network; wherein said pulse generator controls theenergizing and deenergizing of said accelerator and said reflectron; andwherein said detector detects said ions.
 2. An apparatus according toclaim 1, wherein the spatial extent of said accelerator along the axisof said spectrometer is large in comparison to both the initial spatialextent of the analyte ions along the axis of said spectrometer and thespatial extent of the ion accelerator normal to the axis of saidspectrometer.
 3. An apparatus according to claim 1, wherein said networkis designed such that DC potentials applied to said network are dividedby said network in substantially the same manner as are AC potentials.4. An apparatus according to claim 1, wherein said detector ispositioned behind said accelerator.
 5. An apparatus according to claim4, wherein said detector detects ions when said accelerator isdeenergized.
 6. An apparatus according to claim 1, wherein said detectoris positioned behind said reflectron.
 7. An apparatus according to claim6, wherein said detector detects ions when said reflectron isdeenergized.
 8. An apparatus according to claim 1, wherein said detectoris positioned behind both said accelerator and said reflectron.
 9. Anapparatus according to claim 8, wherein said detector detects ions whenone of said accelerator or said reflectron is deenergized.
 10. Anapparatus according to claim 1, wherein: said accelerator and saidreflectron are arranged coaxially with respect to one another; saidaccelerator is deenergized in a pulsed manner; and said detector islocated behind said accelerator for detecting ions while saidaccelerator is deenergized.
 11. An apparatus according to claim 1,wherein: said accelerator and said reflectron are arranged coaxiallywith respect to one another; said reflectron is deenergized in a pulsedmanner; and said detector is located behind said reflectron fordetecting said ions.
 12. A method of analyzing a sample using atime-of-flight mass spectrometer according to claim 10, wherein the timeat which the accelerator is to be deenergized is determined by:determining a first equation which relates the value of the potential(s)on the reflectron to n, the number of times the ions have been reflectedby the reflectron; deriving a second equation based on the firstequation and other fixed parameters in the instrument which can be usedto determine the flight time of ions of the minimum desired m/q fromtheir starting positions, n−1 times reflected through the accelerator,and n times reflected through the reflectron such that said ions areentering the accelerator for the n^(th) time at the end of thedetermined flight time; using said second equation to determine themaximum time at which the accelerator may be pulsed to ground potentialgiven n and a minimum m/q; deriving a third equation based on the firstequation and other fixed parameters in the instrument which can be usedto determine the flight time of ions of the maximum desired m/q fromtheir starting positions, n−1 times reflected through the reflectron,and n−1 times reflected through the accelerator such that said ionswould be exiting the accelerator toward the reflectron for the n^(th)time at the end of the determined flight time; and using said thirdequation to determine the maximum time at which the accelerator may bepulsed to ground potential given n and a maximum m/q.
 13. A method ofanalyzing a sample using a time-of-flight mass spectrometer according toclaim 11, wherein the time at which the reflectron is to be deenergizedis determined by: determining a first equation which relates the valueof the potential(s) on the reflectron to n; deriving a second equationbased on the first equation and other fixed parameters in the instrumentwhich can be used to determine the flight time of ions of the minimumdesired m/q from their starting positions, n times reflected through thereflectron, and n times reflected through the accelerator such that saidions would be arriving at the reflectron for the (n+1)^(th) time at theend of the determined flight time; using said second equation todetermine the maximum time at which the reflectron may be pulsed toground potential given n and a minimum m/q; deriving a third equationbased on the first equation and other fixed parameters in the instrumentwhich can be used to determine the flight time of ions of the maximumdesired m/q from their starting positions, n times reflected through thereflectron, and n−1 times reflected through the accelerator such thatsaid ions would be exiting the reflectron for the n^(th) time at the endof the determined flight time; and using said third equation todetermine the minimum time at which the reflectron may be pulsed toground potential given n and a maximum m/q.
 14. An apparatus accordingto claim 1, wherein: said spectrometer comprises a first and a secondreflectron; said accelerator, said first reflectron and said secondreflectron are arranged with respect to one another such that ions areaccelerated by said accelerator toward said first reflectron arereflected by said first reflectron toward said second reflectron andthereafter reflected by said second reflectron back toward said firstreflectron which reflects said ions back toward said accelerator; andsaid detector is positioned behind one of said accelerator, said firstreflectron or said second reflectron to detect said ions.
 15. Anapparaus according to claim 14, wherein said detector is positionedbehind said accelerator, and wherein said accelerator is deenergized ina pulsed manner to allow said ions be detected by said detector.
 16. Anapparaus according to claim 14, wherein said detector is positionedbehind said first reflectron, and wherein said first reflectron isdeenergized in a pulsed manner to allow said ions be detected by saiddetector.
 17. An apparaus according to claim 14, wherein said detectoris positioned behind said second reflectron, and wherein said secondreflectron is deenergized in a pulsed manner to allow said ions bedetected by said detector.
 18. A method of analyzing a sample using atime-of-flight mass spectrometer according to claim 17, wherein the timeat which the second reflectron is deenergized is found by: determining afirst equation which relates the value of the potential(s) on the firstreflectron to n, the number of times the ions are reflected through thesecond reflectron; deriving a second equation based on the firstequation and other fixed parameters in the instrument which can be usedto determine the flight time of ions of the minimum desired m/q fromtheir starting positions, n times reflected through the secondreflectron, 2n+1 times reflected through the first reflectron, and ntimes reflected through the accelerator such that said ions would bearriving at the second reflectron for the n^(th) time at the end of thedetermined flight time; using said second equation to determine themaximum time at which the second reflectron may be pulsed to groundpotential given n and a minimum m/q; deriving a third equation based onthe first equation and other fixed parameters in the instrument whichcan be used to determine the flight time of ions of the maximum desiredm/q from their starting positions, n times reflected through the secondreflectron, 2n−1 times reflected through the first reflectron, and n−1times reflected through the accelerator such that said ions would beexiting the second reflectron for the n^(th) time at the end of thedetermined flight time; and using said third equation to determine themaximum time at which the second reflectron may be pulsed to groundpotential given n and a maximum m/q.
 19. An apparatus according to claim1, wherein said ion source is an electrospray ionization source.
 20. Anapparatus according to claim 1, wherein said ion source is anatmospheric pressure chemical ionization source.
 21. An apparatusaccording to claim 1, wherein said ion source is a matrix assisted laserdesorption ionization source.
 22. A reflectron for use in massspectrometry which can be energized and deenergized in a pulsed manner,wherein said reflectron comprises at least three conducting electrodesarranged parallel to one another along the axis of said reflectron whichare electrically connected to one another via a resistor-capacitornetwork, wherein the potentials on the electrodes are controlled by thepotentials applied to the inputs of said resistor-capacitor network. 23.A reflectron according to claim 22, wherein the capacitors of saidresistor-capacitor network are formed by said electrodes.
 24. Areflectron according to claim 22, wherein the terminal electrodes ofsaid reflectron comprise planar conducting mesh.
 25. A reflectronaccording to claim 22, wherein said terminal electrodes of saidreflectron comprise planar, conducting, apertured plates.
 26. Areflectron according to claim 22, wherein said terminal electrodes ofsaid reflectron comprise planar, conducting, plates having slits.
 27. Anaccelerator for use in mass spectrometry which can be energized anddeenergized in a pulsed manner, wherein said accelerator comprises atleast three conducting electrodes arranged parallel to one another alongthe axis of said accelerator which are electrically connected to oneanother via a resistor-capacitor network, wherein the capacitors arearranged in parallel to the resistors of said network such that DC andAC potentials applied to the inputs of said network are divided insubstantially the same manner, and wherein the potentials on saidelectrodes are controlled by the potentials applied to the inputs ofsaid network.
 28. An accelerator according to claim 27, wherein thespatial extent of said accelerator in the direction of ion accelerationis large in comparision to both the initial spatial extent of theanalyte ions in the direction of ion acceleration and the spatial extentof the ion accelerator normal to the direction of ion acceleration, 29.An accelerator according to claim 28, where the capacitors of saidnetwork are formed by said electrodes.
 30. An accelerator according toclaim 28, wherein the terminal electrodes of said accelerator compriseplanar conducting mesh.
 31. An accelerator according to claim 28,wherein the terminal electrodes of said accelerator comprise planar,conducting, apertured plates.
 32. An accelerator according to claim 28,wherein the terminal electrodes of said accelerator comprise planar,conducting, plates having slits.
 33. An apparatus for a time-of-flightmass spectrometer, said apparatus comprising: at least one ion producingmeans for generating ions; an ion accelerator; at least one reflectron;at least one pulse generator; at least one ion deflector; and at leastone ion detector; wherein said ions are introduced into said acceleratorto be accelerated along the axis of said spectrometer; wherein each saidreflectron is arranged together with said accelerator such that ions arereflected back and forth between said accelerator and said reflectronwhile said deflector is deenergized; wherein said pulse generatorenergizes and deenergizes said deflector; wherein said detector ispositioned off the axis of said spectrometer; and wherein said deflectoris energized in a pulsed manner to deflect ions into said detector. 34.An apparatus according to claim 33, wherein the spatial extent of saidaccelerator along the axis of said spectrometer is large in comparisionto both the initial spatial extent of the analyte ions along the axis ofsaid spectrometer and the spatial extent of said accelerator normal tothe axis of said spectrometer.
 35. A method for analyzing a sample usinga time-of-flight mass spectrometer, said method comprising the steps of:producing ions from a sample material; introducing said ions into an ionaccelerator; accelerating said ions; reflecting said ions toward saidaccelerator at least one time using a reflectron; reflecting said ionstoward said reflectron at least one time using said accelerator; anddetecting said ions.
 36. A method according to claim 35, wherein saidaccelerator is energized to accelerate said ions to a high kineticenergy.
 37. A method according to claim 36, wherein said accelerator isdeenergized at a predetermined time such that said ions are reflected apredetermined number of times before passing through said acceleratorand into a detector.
 38. A method according to claim 36, wherein saidreflectron is deenergized at a predetermined time such that said ionsare reflected a predetermined number of times before passing throughsaid reflectron and into a detector.
 39. A method of analyzing a sampleusing a time-of-flight mass spectrometer, said method comprising thesteps of: producing ions from a sample material; introducing said ionsinto an ion accelerator; accelerating said ions toward a firstreflectron; reflecting said ions toward a second reflectron at least onetime using said first reflectron; reflecting said ions from said secondreflectron toward said first reflectron at least one time using saidsecond reflectron; reflecting said ions from said first reflectrontoward said accelerator at least one time using said first reflectron;reflecting said ions from said accelerator toward said first reflectronat least one time using said accelerator; and detecting said ions.
 40. Amethod according to claim 39, wherein said ion accelerator is energizedto accelerate said ions to a high kinetic energy.
 41. A method accordingto claim 39, wherein said second reflectron is deenergized at apredetermined time such that said ions are reflected a predeterminednumber of times before passing through said second reflectron and into adetector.
 42. A method of analyzing a sample using a time-of-flight massspectrometer according to claim 1, wherein the optimum potential appliedto the reflectron is determined by: selecting the potential applied tothe accelerator based on practical limitations; determining a firstequation describing the total distance traveled by the ions in the fieldfree regions of the spectrometer as a function of the number of timesthe ions have been reflected by the reflectron; determining a secondequation describing the total effective focal length of the acceleratingand reflecting devices as a function of the number of times the ionshave been reflected by the reflectron; deriving a third equation bysetting the first equation equal to the second equation; determining afourth equation(or set of equations) relating the potential(s) appliedto the reflectron to those applied to the other reflecting andaccelerating devices in the spectrometer; deriving a fifth equation (orset of equations) by substituting said forth equation(s) for thereflectron potential(s) in said third equation; and solving said fifthequation (or set of equations) for the potential(s) applied to thereflectron as a function of the number of times the ions have beenreflected by the reflection.
 43. A method of analyzing a sample using atime-of-flight mass spectrometer according to claim 1, wherein the time,t_(off), at which either said accelerator or said reflectron in front ofsaid detector is deenergized is determined by: measuring the minimum m/qthat can be analyzed for a given n and t_(off) for at least twodifferent values of n or two different values of t_(off); measuring themaximum m/q that can be analyzed for a given n and t_(off) for at leasttwo different values of n or two different values of t_(off); and usingthe four experimental values obtained in steps one and two, to solve theequation: (m/q)_(max)*(a ₁ n+b ₁)² <=t _(off) ²<=(m/q)_(min)*(a ₂ n+b₂)²  simultaneously for the constants a₁,a₂,b₁, an b₂.